1. Field of the Invention
This invention relates to an apparatus for recording and reproducing a digital signal such as a digital video signal or a digital audio signal onto and from a track of a recording medium.
2. Description of the Related Art
An apparatus has been proposed and is disclosed, for example, in EP 0,553,650 A1 wherein an oblique track is formed on a magnetic tape wrapped around a rotary drum and a digital signal such as a digital video signal or a digital audio signal is recorded and reproduced in a time divisional relationship onto or from a sectioned area of the track by means of a rotary head.
FIG. 3 shows a construction of an exemplary one of conventional digital signal recording and reproducing apparatus of the type mentioned. Referring to FIG. 3, the digital signal recording and reproducing apparatus shown includes a recording processing circuit 1 to which a brightness signal Y and a pair of color difference signals PR and PB which constitute analog component video signals are inputted. The signals are processed by various processes including analog to digital conversion, blocking, shuffling, DCT (discrete cosine transform), compression by quantization and variable length coding, and framing by the recording processing circuit 1, and then supplied to a parity generation circuit 3.
The parity generation circuit 3 adds a parity of the product code construction to the data supplied thereto, and the resulted data from the parity generation circuit 3 are supplied to a SYNC and ID generation circuit 4, by which a synchronizing code and an ID are added to the data. The data from the SYNC and ID generation circuit 4 are processed by parallel to serial conversion and conversion into recording codes by a channel encoder 5 and are then amplified by an amplifier 6, whereafter they are supplied to a magnetic head 8 by way of a recording side terminal R of a switching circuit 7 so that they are recorded onto a magnetic tape 9. Here, a digital video signal for one frame is recorded as a plurality of (for example, 10 in the NTSC system, 12 in the PAL system and 20 in the HDTV system) oblique tracks.
It is to be noted that, though not shown, a digital audio signal, a sub code signal and an ATF pilot signal are supplied to the channel encoder 5 and recorded in a time divisional relationship with the digital video signal onto each track on the magnetic tape 9 by the magnetic head 8.
Upon reproduction, data reproduced from the magnetic tape 9 by the magnetic head 8 are supplied by way of a terminal P of the switching circuit 7 on the reproduction side to an amplifier and equalizer 10, by which amplification and correction for the frequency characteristic are performed for the data. The data from the amplifier and equalizer 10 are then processed by decoding into a recording code and serial to parallel conversion by a channel encoder 11 and then supplied to a SYNC and ID detection circuit 12. The ID detection circuit 12 thus detects a synchronizing code and an ID (identification) and supplies them together with the reproduction data to a TBC 13. The TBC 13 removes a time base variation from the data supplied thereto, and the resulted data from the TBC 13 are processed by error correction processing using a product code by an ECC circuit 14 and then supplied to a reproduction processing circuit 16.
The digital video signal supplied to the reproduction processing circuit 16 is processed by various processes including deframing, variable length decoding, dequantization, inverse DCT, deshuffling and deblocking and then converted into analog component video signals by the reproduction processing circuit 16 so that the signals are outputted as a brightness signal Y and a pair of color difference signals PR and PB from the digital signal recording and reproducing apparatus.
It is to be noted that, though not shown, also a digital audio signal and a sub code signal outputted from the ECC circuit 14 are processed similarly. Further, an ATF pilot signal is separated from the output of the channel decoder 11 and supplied to an ATF circuit not shown, by which tracking correction is performed.
FIG. 4 shows an example of a track format employed in the digital signal recording and reproducing apparatus constructed in such a manner as described above. Referring to FIG. 4, the left end of the track shown is the head entrance (incoming) side while the right end is the head exit (outgoing) side. No data are recorded in any one of margins 1 and 2 and IBGs (inter block gaps) which are areas indicated by slanting lines. A pulse signal of a frequency, for example, equal to the bit frequency of data is recorded in a T-amble (track-amble) area added to the front of a first ATF area (ATF1) and amble areas (preamble and postamble areas) added to the opposite ends of a data recording area, and is utilized for locking of a PLL circuit for extraction of bit clocks provided on the reproduction side. The margin 1 and the margin 2 provided at the opposite ends of the track are provided to cope with a case wherein the position at which the track is formed is displaced by jitters. Meanwhile, the IBGs are areas for assuring margins for postrecording.
The first ATF area (ATF 1) and the second ATF area (ATF 2) into which a pilot signal for ATF and a timing synchronizing signal for postrecording are recorded are provided adjacent the margins 1 and 2, respectively. Further, in order of the scanning direction of a head from the first ATF area, a recording area for an audio signal, a recording area for a video signal, a recording area for sub codes and the second AFT area are provided. Data such as an index and a time code for a high speed search are recorded in the sub code recording area. The numeral indicating the length of each area represents the number of bytes.
FIGS. 5(a) to 5(c) show an example of a one synchronizing block and framing formats of a digital video signal and a digital audio signal. In FIGS. 5(a) to 5(c), each numeral represents the number of bytes.
In particular, FIG. 5(a) shows one synchronizing block. As seen in FIG. 5(a), a horizontal parity of 8 bytes is added to the rear of digital video data or digital audio data of 78 bytes while a synchronizing code of 2 bytes and an ID of 3 bytes are added to the front of the digital video data or digital audio data. The ID has information for identification of a synchronizing block number and a type of the data. Subsequently, a method of forming such synchronizing block will be described briefly.
FIG. 5(b) shows a framing format for a digital video signal. Referring to FIG. 5(b), video data (including quantized information and so forth) of 78 bytes in a horizontal direction are arranged by 45 in a vertical direction by the recording processing circuit 1, and horizontal parities C1 and vertical parities C2 are added to the data by the parity generation circuit 3. Further, synchronizing codes and IDs are added to the data by the SYNC and ID generation circuit 4. Then, the resulted data are converted from parallel data into serial data by the channel encoder 5 and then recorded for each synchronizing block shown in FIG. 5(a). Three frames in FIG. 5(b) make a digital video signal for one track.
Similarly, FIG. 5(c) shows a framing format for a digital audio signal. One frame shown in FIG. 5(c) makes a digital audio signal for one track.
An ATV (Advanced Television) system wherein HDTV (high definition television) broadcasting is realized with digital transmission is investigated. In such ATV system, an HDTV signal of approximately 1.2 Gbit/sec is compressed to 18 to 25 Mbit/sec by motion compensation inter-frame prediction, DCT, quantization and variable length coding, and synchronizing codes, headers and parities are added to the compressed signal. Then, the resulted signal is converted into a signal of a format of a packet of a fixed length of, for example, 155 or 167 bytes to transmit the same.
Meanwhile, as an international standard system for coding moving picture data at a high efficiency, an MPEG (Moving Picture Experts Group) system has been proposed. Also in the MPEG system, a picture signal is compressed to 5 to 10 Mbit/sec by motion compensation inter-frame prediction, DCT, quantization and variable length coding.
When it is tried to record digital data of the ATV system or digital data compressed in accordance with the MPEG system using such a conventional digital signal recording and reproducing apparatus as described above, if the digital data are recorded, for example, into a recording area (1,274 bytes) for a digital audio signal and another recording area (13,377 bytes) for a digital video signal for each track, then the capacity (200 bytes) of the IBG and amble portions becomes wasteful. On the other hand, if it is tried to record data also into the IBG and amble portions, then it is necessary to displace the positions of the sub code recording area and the ATF areas.